Saturday, June 28. 2014

The main thing with Cambridge, Massachusetts, is its diversity. The big universities are the focus of a vibrant multi-ethnic community, with Harvard Square and Central Square its centers of gravity. The two squares are a joy to visit, as they host many small bookstores (revolution books yay!) and restaurants, still somehow able to survive despite the gentrification that followed the housing bubble. Among our favorite places in Harvard Square, however, there is a very special yoga studio.

The studio is special because of its owner, Jesse. We started frequenting him when the studio was at its infancy, in a small basement room in Harvard Square just big enough for a few mats along one wall (I once managed to actually dent the wall, which tells a lot about my "flexibility"). Now the studio has grown a lot, opened a second downtown location, a "donation" studio (you pay what you can) a no-kill animal shelter, and much more. But Jesse is still the same Jesse, and we were lucky of being able to meet him for a chat during our last Boston visit. And as usual with Jesse, we talked about science and the nature of reality as described by physics (Jesse is more up to date about science news than me).

Riverfest in Cambridge

All this came back to my mind a couple of days ago as I read a few articles celebrating the 50 years since the publication of the Bell's Theorem, that more than any other results in modern physics is at the base of our attempts to understand reality. The theorem applies to quantum mechanics, the foundational theory of modern physics. Developed at the beginning of the XX century, quantum mechanics describes the behaviour of subatomic particles and forces. It replaces the determinisms of classical mechanics (where everything is predictable with clockwork precision) with a probabilistic approach. With quantum mechanics you cannot foretell where a particle will be at a given time, only estimate the probability that it will be there, or at any other location. This view, which is based on the work of Niels Bohr and the "Copenhagen" school, has been found to be inherently unsatisfactory to many scientists, including Albert Einstein when he proclaimed that "God doesn't play dice with the world". The favorite loophole invoked by the critics was to assume that quantum mechanics was an incomplete theory, e.g. that there was some missing equation with hidden variables that, if measured, would predict the exact outcome of any physics experiment. Bell's theorem closed this loophole, by showing that any physical theory (including quantum mechanics) cannot have hidden local variables. As a consequence, the world is either inherently random (and the exact result of any experiment is unknowable until somebody measures its outcome), or physics is non-local.

Losing locality, however, is as bad for common sense as much as believing in a random universe. If physics is non-local then anything anywhere in the universe can in principle affect, instantaneously, the outcome of a physics experiment. This contradiction was famously popularized by Einstein, again, when he suggested that the loss of locality would lead to a "spooky action at a distance", where entangled particles would have the mysterious ability to determine each other status instantaneously as if they could communicate "telepathically". While this paradox was proposed to show the absurdity of a non-local universe, reality is full of surprises, and quantum entanglement (as this spooky action at a distance is called) was indeed confirmed in several experiment starting from 1972. The story of how these experiments is interesting in itself, and beautifully narrated in the book "How The Hippies Saved Physics". This the story of a band of physicists coming from the counterculture of the fabulous sixties set out to understand the deeper meaning of physics in between acid trips and paranormal experiments. While they did not prove the existence of telepathy and the new age idea of the oneness of the universe, they did demonstrate the reality of quantum entanglement, inventing in the process the field of quantum encryption and quantum computing.

While quantum entanglement seems to suggest that physics is indeed not local, superluminal communication is still prohibited by Einstein special relativity. The true meaning of this spooky action at a distance, as a consequence, is quite obscure. Living in a fundamentally unpredictable universe is however also very unsatisfactory, despite the sci-fi appeal of multi world theories where all possible quantum solutions do happens at the same time, just in different "parallel" universes. A century after being discovered, we casually use quantum mechanics without even noticing, every time we turn on the switch of any electronic devices whose semiconductor technology is based on the weird nature of quantum mechanics. Yet, like children playing with magic toys from alien civilizations, we are still far from understanding the deep principles upon which our gadgets are based and, fifty years after the publication of Bell's theorem, the true nature of our quantum universe.